The treatment of a cancer is one of applications of radiation. Recently, a particle beam treatment in which a heavy particle beam such as a proton beam or a carbon beam is irradiated onto the cancer cell has been attracted attention. First of all, the characteristic of a particle beam irradiation in which a particle beam is irradiated to kill a cancer cell will be described. In a case where various kinds of irradiation beams are irradiated onto a human body, the dose distribution of the irradiation beam in the human body changes as shown in FIG. 15. As shown in FIG. 15, among various kinds of irradiations, a photon beam such as an X-ray or a gamma ray, has a relative dose which becomes maximum in a portion close to the surface of the body, and is decreased as the depth from the surface of the body is increased. On the other hand, a particle beam, such as a proton beam or a carbon beam, has a relative dose which has a peak value at a position where the beam stops at a deep portion from the surface of the body, that is, immediately before the range of the particle beam. This peak value is called the Bragg Peak (BP).
Particle beam cancer treatment is such that this Bragg peak BP is irradiated to a tumor formed in a human organ and the treatment of the cancer is performed. In addition to the cancer, it can also be used for a case where a deep portion of a body is treated. A region to be treated, including a tumor, is generally called an irradiation target. The position of the Bragg peak BP is determined by the energy of an irradiated particle beam, and as the energy of the particle beam becomes higher, the Bragg peak BP is formed at a deeper position. In the particle beam treatment, it is required to provide uniform distribution of dose of a particle beam in whole region of an irradiation target. In order to give the Bragg peak BP to the whole region of the irradiation target, “spread of the irradiation volume” of the particle beam is performed.
This “spread of the irradiation volume” is performed in three directions of an X-axis, a Y-axis and a Z axis perpendicular to each other. When an irradiation direction of a particle beam is set to be a direction of the Z-axis, “spread of the irradiation volume” is first performed so as to spread the irradiation field in the X-axis and Y-axis directions, and since the irradiation field spread is performed in a lateral direction perpendicular to a depth direction, it is generally called the irradiation field spread. The second “spread of irradiation volume” is performed in the Z-axis, and it is called the irradiation volume spread in a depth direction.
The irradiation volume spread in a depth direction is performed to spread the Bragg peak BP in an irradiation direction of a particle beam to a depth direction since the width of the Bragg peak BP in an irradiation direction of a particle beam is narrow as compared with the extent of an irradiation target in a depth direction. On the other hand, the irradiation field spread in a lateral direction is performed to spread the irradiation field in the Bragg peak BP in a direction perpendicular to an irradiation direction since the diameter of a particle beam, which is accelerated by an accelerator generally, is smaller than the size of an irradiation target in a direction perpendicular to an irradiation direction. With respect to the irradiation volume spread in a depth direction and the irradiation field spread in a lateral direction, various kinds of methods have been proposed so far. Recently, Scanning Irradiation has attracted attention.
In Scanning Irradiation, as an irradiation field spread method in a lateral direction, there is a method in which a deflection electromagnet provided at the upstream portion of a particle beam irradiation part of a particle beam treatment device is used to scan a particle beam in the XY plane, and an irradiation position of the particle beam is moved with the lapse of time to obtain a wide irradiation field. In this method, a uniform dose distribution can be obtained by suitably overlapping adjacent irradiation spots of pencil beams having a small diameter. Scanning methods of a pencil beam include a raster method of performing scanning continuously with respect to time, a spot method of performing a step-like scanning with respect to time and a method combining the raster method and the spot method.
As the irradiation volume spread method in the depth direction, there is a method in which the energy of a particle beam itself which is irradiated from a particle beam treatment device is controlled. In this method, the energy of a particle beam is controlled by changing the acceleration energy of an accelerator which accelerates the particle beam, or the energy of a particle beam is changed by inserting a tool called a range shifter so as to cross the particle beam. There is also a method in which both the control by the accelerator and the range filter are used.
In the irradiation volume spread method in the depth direction, a particle beam is made to have the energy of specified intensity, after one of irradiation layers of an irradiation target volume is irradiated with the Bragg peak BP, the energy of the particle beam is changed, and next irradiation layer of the irradiation target volume is irradiated with the Bragg peak BP. Such operation is repeated plural times so as for plural irradiation layers to be irradiated with the Bragg peak BP of the particle beam. Consequently, the Spread-out Bragg peak SOBP having a desired width in a beam irradiation direction can be obtained. (For example, Patent Document 1)
A particle beam irradiation method which is made by combining the irradiation field spread method in a lateral direction and the irradiation volume spread method in a depth direction is called Scanning Irradiation.
Further, a method in which irradiation is performed at the same spot position for plural times with time-wise dividing at each irradiation layer so as to compensate the deviation of irradiation caused by moving of a position of affected site due to a patient's breathing, is proposed (For example, Patent Document 2, FIG. 11). Further, in the Patent Document 2, a technique, for controlling the irradiation dose by synchronizing with a breathing phase in consideration of the moving of an affected site caused by breathing, is also proposed.